Author
Topic: Minnie on the cheap! (Read 6157 times)

A few months ago I went to an Autojumble and found a model boiler for sale for £5! I snapped it up and took it home with glee. After some investigation I found that it was the boiler for the Minnie traction engine, but, a big but, it was made in steel. I thought about it and realised that if I made this engine I would probably only steam it once or twice and mostly run it on compressed air. So a steel boiler would be fine. I am short on cash so have to make it as cheaply as possible. The commercially available castings are quite expensive for a pauper! For example, the aluminium castings for the rear wheel rims are £70, I made them for the cost of the gas for my furnace. These will appear later. My current problem is the castings for the bearings, the commercially available ones are bronze. What type of bronze? Can I cast them myself? It would be nice to make accurate castings. I have made the patterns in aluminium and have made the moulds in plaster of paris.Bronze, there is bronze that is an alloy of copper and tin or an alloy of copper and aluminium. Lead free solder (99c) is almost pure tin, I could use that. There are so many variables. Whatever I use will not be perfect but will probably be good enough for what I want to do. Will my mild steel crucible be enough for the job? Time will tell.

The Copper Development Association published a comprehensive book entitled 'Aluminium Bronze'. I understand that all their publications, including that one, are now available on-line.

One bit I remember from the chapter about casting Aluminium Bronze is that if there are multiple paths around the mould cavity, e.g. around the core or a branched feeder, the two streams of molten metal won't merge properly when they meet. I guess it's because the stream of molten metal has an oxide skin. I also seem to remember a comparison with pouring beer into a glass!

Apart from that, aluminium bronze works well immersed in water and is fairly easy to machine. All that from memory, I suggest that you seek the .pdf.

Logged

Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest change-note!

I have cast the wheel hubs in aluminium and sleeved them but I just wanted an excuse to try casting bronze. It is also quite interesting to try using a plaster mould taken from machined metal patterns that are more accurate than what I could produce using wood.

Some more parts, the wheel rims. I can't understand why the castings for the rear rims are so expensive (£70)! They cost me next to nothing to cast them myself.The strakes on the hind wheels were a bit tedious to make but riveting them was fun.Also the wheel hubs and associated bits. The slots for the spokes are a bit rough, I made my own milling cutter, it didn't keep its edge, but it won't matter as the hub covers will hide it all.

Oops, sorry, that's the wrong publication. Still, if you Google 'Copper Development Association', there are several publications on Aluminium Bronze. It's a bit near bed-time right now - I'll have a look for my hard copy tomorrow and post here again with the CDA publication number.

Logged

Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest change-note!

I gave up on the idea of making bronze castings for the bearings as I could not source any copper. Instead I thought that I would cast them in aluminium and sleeve them in a more hard wearing metal. The castings in my expensive plaster moulds were a failure. I believe that as the plaster was cold it caused the metal to shrink away from them. My precise sharp cornered patterns were translated into rounded cornered castings!What a waste of money, £12 for the plaster.

So instead, I fabricated to bearings in steel. I do not have any silver solder so I araldited the parts together. They look ok, remember, this is intended to be a display model rather than a fully functioning one.

The plan calls for ground steel for the shafts, I decided to turn them from oversize metals. The longest shaft is 12mm dia x 200mm long. I turned it between centres and initially it turned out oversize at the tailstock end but a bash with a hammer on the tailstock ended up with it being parallel over the whole length. I am quite pleased with that. After that the other shafts, only 100mm long, were a doddle.

I notice that I am not getting much response to this thread, I suppose it is because there are many Minnies that have been built and nobody is interested any more.I have a question. I want to cut the gears myself, I have never done it before. It appears that there are two approaches, I could hob the gears which seems to only need one cutter or I could use a set of cutters to cut individual teeth. A ready made hob costs approximately £45 the material to make it costs £25. It hardly seems worth the risk of cocking it up so I would prefer to buy a ready made one. The individual cutters are £25 each, I would need 4 totalling £100. Buying ready made gears is starting to sound attractive!Here is my question, would my milling machine with its 1/2 Horsepower motor have sufficient power to use a hob?